Snowmobile suspension system (CA)

Canadian Application Publication

Title (French)

Suspension de motoneige

Abstract (English)

A snowmobile suspension system used to suspend the drive belt of a snowmobile between extended and retracted position is described herein. The snowmobile suspension system is provided with a pair of side rails to guide the drive belt, a pair of suspension arms each having a proximate end pivotally mounted to the snowmobile and a distal end pivotally mounted to both side rails, shock absorbers and compression springs provided between the side rails and the snowmobile to bias the side rails towards their extended position, and a progressive spring assembly mounted to the side rails to prevent forceful contact between the side rails and the undersurface of the snowmobile. The side rails are advantageously made of composite material and present a concave undersurface. The pivotal attachment of the distal end of the rear suspension arm to the side rails is advantageously made via an adjustable cam assembly allowing the user to modify the characteristics of the snowmobile suspension system.

Language of Filing

Attorney, Agent or Firm

Also Published As

CA 02282787 1999-09-171 TITLE OF THE INVENTION SNOWMOBILE SUSPENSION SYSTEM FIELD OF THE INVENTION The present invention relates to suspension systems. More specifically, the present invention is concerned with a snowmobilesuspension system. BACKGROUND OF THE INVENTIONIt is well known in the art to suspend the drive belt of asnowmobile below the body thereof for suspension movements. These suspension systems are usually provided with apair of side rails to guide the drive belt, a pair of suspension arms eachhaving a proximate end pivotally mounted to the snowmobile and a distalend pivotally mounted to both side rails. The side rails, suspension armsand the snowmobile are so configured as to form a deformableparallelogram, thereby allowing suspension movements of the side railswith respect to the snowmobile between a fully extended and a fullyretracted position. Shock absorbers and compression springs are alsoprovided between the side rails and the snowmobile to bias the side railstowards their extended position.

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2 Such conventional snowmobile suspension systemssuffer from many drawbacks. A first drawback relates to the materialused to make the side rails. Indeed, side rails are usually made ofaluminum which yields relatively heavy and complicated to manufactureside rails. Furthermore, aluminum side rails have other drawbacksrelated to the nature of the material such as, for example, poor resistancetoughness and low fatigue life. A second drawback of the conventional snowmobilesuspension systems is revealed when a force, overcoming the forcesapplied by the shock absorbers and the springs, causes the side rails togo from their fully extended position to their fully retracted position. Whenthis is the case, the side rails will hit the underside of the snowmobile,which may damage the vehicle and/or endanger the driver. To overcome this drawback, commercially availablesnowmobile suspension systems are provided with a piece of resilientmaterial, for example, rubber, strategically positioned to intercept the siderails before they contact the underside of the snowmobile. The contactis therefore much less violent since the resilient material absorbs aportion of the energy of the impact. However, the contact between theside rails and the rubber piece still causes undesired sensations to thedriver. A third drawback is the lack of adjustability of thesuspension characteristics or the complexity of adjustment of thesecharacteristics. Indeed, conventionally, the pivotal attachments securingthe distal end of the rear suspension arm to the side rails and the shock

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3absorbers to the side rails are at predetermined fixed positions whichresults, for a given configuration, in predetermined suspensioncharacteristics. This is a drawback since it removes the possibility for thedriver to select different suspension characteristics for different type ofrides. To address this problem, many adjustable snowmobilesuspension systems have been proposed. For example, United States Patent N° 5,692,579 issued on December 2nd 1997, naming Keith W. Peppel et al. as inventors and entitled « Adjustable Snowmobile Track Suspension » discloses a system provided with a rear suspension armhaving a distal end which is so mounted to the side rails that limitedlongitudinal movements of the distal end may be achieved. The systemsalso include adjustable limits allowing the characteristics of thesuspension to be adjusted by the user. While Peppel’s suspension is an improvement in termsof adjustability, it involves many mechanical elements to allow thelongitudinal movements and to limit these movements. Furthermore, theuser needs some tools to loose the fasteners, rotate the rectangular limitsand to tighten the fasteners. OBJECTS OF THE INVENTION An object of the present invention is therefore to providean improved snowmobile suspension system.

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4 SUMMARY OF THE INVENTION More specifically, in accordance with the presentinvention, there is provided A snowmobile suspension system configuredto suspend a drive track to a snowmobile; the suspension systemcomprising:a front suspension arm having a proximate end pivotallymounted to the snowmobile; the front suspension arm having a distal end;a rear suspension arm having a proximate end pivotallymounted to the snowmobile; the rear suspension arm having a distal end;a pair of side rails; each side rail being pivotally mountedto the distal ends of the front and rear suspensions for suspensionmovements between a retracted position where the side rails are adjacentto the snowmobile and an extended position; each side rail being madeof composite material and provided with a concave undersurface; anda biasing assembly pivotally mounted to the snowmobileand to both side rails; the biasing assembly biasing both side railstowards the extended position. According to a second aspect of the present invention,there is provided a A snowmobile suspension system configured tosuspend a drive track to a snowmobile; the suspension systemcomprising:a front suspension arm having a proximate end pivotallymounted to the snowmobile; the front suspension arm having a distal end;a rear suspension arm having a proximate end pivotallymounted to the snowmobile; the rear suspension arm having a distal end;

CA 02282787 1999-09-17a pair of side rails; each side rail being pivotally mountedto the distal ends of the front and rear suspensions for suspensionmovements between a retracted position where the side rails are adjacentto the snowmobile and an extended position;

5 a biasing assembly pivotally mounted to the snowmobileand to both side rails; said biasing assembly biasing both side railstowards the extended position; anda progressive spring assembly so mounted to the pairof side rails as to contact at least one of the front and rear suspensionarms when the pair of side rails nears the retracted position therebyabsorbing excess energy from the side rails to prevent forceful contactbetween the side rails and the snowmobile. According to a third aspect of the present invention,there is provided a A snowmobile suspension system configured tosuspend a drive track to a snowmobile; the suspension systemcomprising:a front suspension arm having a proximate end pivotallymounted to the snowmobile; the front suspension arm having a distal end;a rear suspension arm having a proximate end pivotallymounted to the snowmobile; the rear suspension arm having a distal end;first and second longitudinal side rails; each longitudinalside rail being pivotally mounted to the distal end of the front suspensionarm; each longitudinal side rail having a generally transversal rear pivotalaxis and a semi-circular channel coaxial with the rear pivotal axis; thesemi-circular channel having opposite ends;

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6a linkage assembly connecting the distal end of the rearsuspension arm to the pair of side rails; the linkage assembly including,for each side rail:an adjustable cam assembly so mounted to the side railas to pivot about the rear pivotal axis; the camassembly having an off-center arm mounting apertureto which the distal end of the rear suspension arm ispivotally mounted; andat least one adjustable abutment element so mountedto the cam assembly as to extend in the semi-circularchannel; the abutment element limiting the pivotingmovements of the cam assembly by abutting the endsof the semi-circular channel; anda biasing assembly pivotally mounted to the snowmobileand to both side rails; the biasing assembly biasing both side railstowards an extended position. Other objects, advantages and features of the presentinvention will become more apparent upon reading of the following nonrestrictive description of preferred embodiments thereof, given by way ofexample only with reference to the accompanying drawings. BRIEF DESCRIPTION OF THE DRAWINGSIn the appended drawings:

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7 Figure 1 is a sectional side elevational view illustratinga snowmobile suspension system according to an embodiment of thepresent invention; the snowmobile suspension system being shownmounted to a snowmobile; Figure 2 is a sectional side elevational view of thesnowmobile suspension system of Figure 1; Figure 3 is a side elevational view of one of the side railsof the snowmobile suspension system of Figure 1; Figure 4 is a plan view taken along line 4-4 of Figure 1; Figure 5 is a side elevational view illustrating thesnowmobile suspension system of Figure 1 in a resting state; Figure 6 is a side elevational view illustrating thesnowmobile suspension system of Figure 1 when the snowmobile isunder a normal acceleration; Figure 7 is a side elevational view illustrating thesnowmobile suspension system of Figure 1 when the snowmobile isunder a strong acceleration; Figure 8 is a side elevational view illustrating thesnowmobile suspension system of Figure 1 when the front portion of thesuspension system hits a bump; and

CA 02282787 1999-09-17 Figure 9 is a side elevational view illustrating thesnowmobile suspension system of Figure 1 in a retracted position. DESCRIPTION OF THE PREFERRED EMBODIMENT Turning now to Figures 1 to 4 of the appended drawings,a snowmobile suspension system 10 according to an embodiment of thepresent invention will be described. In Figure 1, the snowmobile suspension system 10 isillustrated suspended from the underside of a snowmobile 12. As will bedescribed hereinbelow, the snowmobile suspension system 10 may movefrom a fully extended position to a fully retracted position. As can be better seen from Figure 2, the snowmobilesuspension system 10 is provided with a pair of side rails 14 and 16 (onlyone shown) to guide a drive belt (not shown), front and rear identicalsuspension arms 18 and 20, a biasing assembly 22 and a progressivespring assembly 24. The side rails 14 and 16 are identical. They areadvantageously made of a Ultra-High Molecular Weight (UHMW) materialsuch as, for example, Ultra-High Molecular Weight Polyethylene (UHMW PE) manufactured under the tradename Tivarc~ 1000 by Poly Hi Solidur,a division of Menasha Corporation.

CA 02282787 1999-09-179 Since the side rails 14 and 16 are made of a polymericmaterial, they are advantageously provided with a concave undersurface(see undersurface 26 in Figure 3). Therefore, when pressure is appliedin the direction of arrow 28, the side rails 14 and 16 will have thetendency to flatten to therefore present a generally flat undersurface andincrease the stiffness of the side rails. It has been found that when Tivart~ 1000 is used, a side rail 14 of about 46 inches (about 1.17 meters)operates in a satisfactory manner when the undersurface 26 is generallysemicircular and has a radius of about 400 inches (about 10.16 meters). Of course, the above measurements are given as an example only anddepend, amongst others, on the shape of the rails, the type of materialused and the type of snowmobile used. Another way of looking at the undersurface 26 of the rail14 is that it presents an inward central deflection of about’/4 of an inch(about 0.006 meters). Of course, the undersurface 26 could presentanother profile, as long as the undersurface is not straight. For example,2 straight surfaces (not shown) could be joined to present a centraldeflection of about’/4 inch. As will be apparent to one skilled in the art, the term« concave », as used in the present description and in the appended claims,is to be construed as meaning any surface that presents an inwarddeflection, not necessarily semi-circular in shape. The rail 14 includes a plurality of thinner portions 29 soconfigured, sized and positioned as to decrease the overall weight of therail 14 without significantly decreasing its stiffness.

CA 02282787 1999-09-17 It has been found that rails following the shapeillustrated in the appended figures, when made of Tivar~ 1000, offersimilar performances as conventional aluminum rails while being lighterand free of the above-noted drawbacks of conventional aluminum side5 rails. The weight required to flatten the undersurface 26 of the railsdescribed hereinabove is about 500 pounds (about 227 kg). The front and rear suspension arms 18 and 20 define,with the side rails 14 and 16 and with the underside of the snowmobile10 12, a parallelogram, allowing suspension movements of the side rails 14and 16 with respect to the snowmobile between fully extended and fullyretracted positions. The front suspension arm 18 is generally Y-shaped (see Figure 4) and includes a proximate portion 30 provided with a cylindricalmounting rod 32 having opposite projections 34 and 36 configured andsized to be pivotally mounted to corresponding cylindrical apertures (notshown) of the snowmobile. The front suspension arm 18 also includes adistal portion 38 provided with a cylindrical mounting rod 40 havingopposite projections 42 and 44 configured and sized to be pivotallymounted to corresponding cylindrical apertures of respective side rails 14and 16. Similarly, the rear suspension arm 20 is generally Y-shaped (see Figure 4) and includes a proximate portion 46 provided witha cylindrical mounting rod 48 having opposite projections 50 and 52configured and sized to be pivotally mounted to corresponding cylindricalapertures (not shown) of the snowmobile. The rear suspension arm 20

CA 02282787 1999-09-1711also includes a distal portion 54 provided with a cylindrical mounting rod56. The mounting rod 56 is pivotally mounted to both side rails 14 and 16via identical adjustable cam assemblies 58, 60. For concision purposes,only the adjustable cam assembly 58 will be described hereinbelow. It is to be noted that since the front and rear suspensionarms 18 and 20 are advantageously made of Tivar~ 1000, which is a self-lubricating material having a low coefficient of friction, other frictionreducing material is not required between the mounting rods 32, 40 andthe front suspension arm 18 and between the mounting rods 48, 56 andthe rear suspension arm 20. The adjustable cam assembly 58 includes first andsecond semi-circular disks 62, 64 so mounted to an aperture 66 (Figure3) of the side rail 14 as to pivot about a central first pivotal axis 67. Thesemi-circular disks are interconnected via a fastener 68 and may includecylindrical projecting portions (not shown) configured and sized to enterthe aperture 66. Again, since Tivar~ 1000 is a self lubricating materialhaving a low coefficient of friction, other friction reducing material is notrequired between the projection and the aperture 66. Each semi-circular disk 62 and 64 includes a series ofcircular apertures 70a-70h equidistant from the first pivotal axis 67. Theapertures 70a-70h are therefore positioned on an hypothetical semicircular line. Each disk 62 and 64 also includes an offset mountingaperture 72 to which the distal end 54 of the rear suspension arm 20 may

CA 02282787 1999-09-1712be pivotally mounted. As can be better seen from Figure 4, the mountingrod 56 is mounted to the adjustable cam assemblies 58 and 60 viafasteners 74, 76 so as to pivot about a second pivotal axis 78. Returning to Figure 3, the side rail 14 is provided with asemi-circular slot 80 aligned with the hypothetical semi-circular linedefined by the apertures 70a-70h. The slot 80 has a front end 82 and arear end 84 (see Figure 3) and allows a front adjusting pin 86 and a rearadjusting pin 88 to be inserted in respective apertures 70a-70h of bothdisks 62 and 64 while extending through the slot 80. As will be furtherdescribed hereinbelow, the adjusting pins 86 and 88 respectively limit thepivotal movement of the adjustable cam assembly 58 in clockwise andcounterclockwise directions, since the adjustable cam assembly 58 maynot pivot further when one of the adjusting pin contacts one of the endsof the slot. Advantageously, the front adjusting pin 86 is inserted in oneof the first five apertures 70a-70e, while the rear adjusting pin 88 isinserted in one of the last three apertures 70f-70h. The biasing assembly 22 includes a pair of shockabsorbers 90, 92 each having a proximate end mounted to the mountingrod 48 of the rear suspension arm 20 and a distal end mounted to anadjustable spacer rod 94. The spacer rod 94 is slidably mounted in arectangular aperture 96 of the rails (see Figure 3) thereby allowing limitedlongitudinal adjustment of the mounting position of the distal end of theshock absorbers 90, 92. The biasing assembly 22 also includescompression springs 98, 100 conventionally provided around the shockabsorbers 90, 92, respectively.

CA 02282787 1999-09-1713 Of course, the biasing assembly 22 is provided betweenthe side rails and the snowmobile to bias the side rails towards theirextended position. As will be apparent to one skilled in the art, thelongitudinal adjustment of the spacer rod 94 enables the user to modifythe stiffness of the biasing assembly 22. The snowmobile suspension system 10 also includes aslide bar 102 having a proximate end 104 pivotally mounted to themounting rod 32 of the front suspension arm 18 and a distal end 106provided with an elongated slot 108 mounted to a spacer rod 110interconnecting the side rails 14 and 16. Of course, as will be apparentto one skilled in the art, the purpose of the slide bar 102 is to limit theopening of the front suspension arm 18. The progressive spring assembly 24 includes twoseparate springs 112 and 114 having a rising strength rate. Each springis mounted in a semicircular opening 116 of the rails (see Figure 3) andincludes a generally L-shaped leg 118 inserted in an aperture 120 of therails to prevent rotation of the spring in the semi-circular opening 116. Each spring also includes a generally straight contact portion 122 sopositioned, configured and sized as to contact the rear suspension arm20 when the snowmobile suspension system 10 is near its fully retractedposition. The purpose of the progressive spring assembly 24 is toprevent forceful contact between the elements of the snowmobilesuspension system 10 and the underside of the snowmobile 12 byprogressively absorbing energy as the snowmobile suspension system 10

CA 02282787 1999-09-1714nears its fully retracted position. This energy absorption will slow themovement of the snowmobile suspension system 10, therefore minimizingthe chances of contact. It is to be noted that the snowmobile suspension systemincludes other elements that are not relevant to the present invention,such as, for example, spacer rods 124 and endless track contactingwheels 126. These elements will not be further discussed herein sincethey are believed well known to one skilled in the art. Turning now more specifically to Figures 5-9 of theappended drawings, the operation of the snowmobile suspension system10 according to an embodiment of the present invention will be described. Figure 5 illustrates the snowmobile suspension system10 mounted to the snowmobile 12 when the snowmobile suspensionsystem is in a resting state. Therefore, only the weight of the snowmobile12 and of the user (not shown) applied downward pressure onto thesnowmobile suspension system 10 which is thus in its fully extendedoperating position. It is to be noted that when the snowmobile suspensionsystem 10 is in this position, the rear adjusting pin 88, which is insertedin the aperture 70f, abuts the rear end 84 of the slot 80. The camassembly 58 has therefore pivoted about axis 67 (see arrow 128) withrespect to the position of the snowmobile suspension system 10 shownin Figure 2. However, even if more weight is applied to the snowmobilesuspension system 10, there will be no further counterclockwise rotation

CA 02282787 1999-09-17of the cam assembly 58 since there is contact between the rear adjustingpin 88 and the rear end 84 of the slot 80. Therefore, if more weight isapplied to the snowmobile suspension system 10, the parallelogramdefined by the underside of the snowmobile 12, the suspension arms 185 and 20, and the rail 14 will flatten. It is to be noted that if the rear adjustment pin 88 hadbeen inserted in one of the aperture 70g or 70h, the position of thesnowmobile 12 at its resting state would have been different from the one10 illustrated in Figure 5. Indeed, since the weight of the snowmobile issufficient to ensure that the rear adjustment pin 88 contacts the rear end84 of the slot 80, the insertion of the adjustment pin 88 in anotheraperture change the dynamic of the snowmobile suspension system 10. More specifically, when the adjustment pin 88 is inserted in the aperture15 70h the ride is smoother than if the adjustment pin 88 is inserted in theaperture 70f since the insertion in aperture 70h causes a pre-pivotingaction of the cam assembly 58, therefore shortening the effective lengthof the rear suspension arm 20 which, as will be described hereinafter,increases the pulling action (see arrow 160 in Figure 8) when thesnowmobile hits a bump. Turning now to Figure 6 of the appended drawings, theoperation of snowmobile suspension system 10 when the snowmobile 12is under normal forward acceleration (see arrow 130) will be described. When this is the case, the front portion of the snowmobile 12 is lifted fromthe ground while the rear portion of the snowmobile 12 drops towards theground (see arrow 132). The acceleration forces cause a downwardforce on the rear suspension arm 20 (se arrows 134 and 138) that cause

CA 02282787 1999-09-1716the clockwise rotation (see arrow 140) of the cam assembly 58 aboutpivot axis 67. This clockwise rotation causes the rearward displacementof the pivotal axis 78 therefore modifying the shape of the parallelogramdefined by the front and rear suspension arms 18 and 20, the side rails14 and 16 and the underside of the snowmobile 12. Hence, thedownward movement of the rear portion of the snowmobile 12 has beenabsorbed by the pivotal movement of the cam assembly 58 withoutrequiring any movement of the front suspension arm. It is to be noted that the front adjustment pin 86 does notcontact the front end 82 of the slot 80. Once this contact is made, furtherincrease in acceleration is translated in a general movement of thesnowmobile suspension system 10 towards its retracted position. Indeed,if the pin 86 contacts the end 82, further pivotal movement of the rearsuspension arm 20 about axis 67 is prevented, thus forcing the pivotalmovement of the rear suspension arm 20 about pivotal axis 78. It is also to be noted that when the adjustment pin 86contacts the front end 82 of the slot 80, further increase in accelerationdoes not cause further upward movement of the front portion of thesnowmobile 12 but cause only further downward movement of the rearportion of the snowmobile 12. In Figure 7, the front adjustment pin is shown insertedin aperture 70e while the snowmobile is shown under strong acceleration(see arrow 142). Again, the front portion of the snowmobile 12 is liftedfrom the ground while the rear portion of the snowmobile 12 dropstowards the ground (see arrow 144).

CA 02282787 1999-09-1717 Under a strong acceleration, the force on thesnowmobile suspension system 10 (see arrow 146) will cause the pivotingaction of the distal ends of the suspension arms (see arrows 148 and150, respectively) to thereby move the snowmobile suspension system10 towards its retracted position. It is to be noted that the undersurface 26 of the rail 14has lost its deflection since the load transferred to the rail 14 by thebiasing assembly is great. It is also to be noted that when the adjustment pin 86 isinserted in the aperture 70e the weight shift effect is greater than if theadjustment pin 86 is inserted in the aperture 70a since the insertion inaperture 70e allows a greater upward movement of the front portion of thesnowmobile 12 than would the insertion in aperture 70a. Figure 8 of the appended drawings illustrates the frontportion of the rail 14 moved upwardly (see arrow 152) by a bump 154during forward movement of the snowmobile 12 (see arrow 156). Whenthis is the case, the rail 14 is moved rearwardly (see arrow 158), therebycausing the rear arm 20 to be pulled (see arrow 160), that, in turn causesthe rear portion of the snowmobile 12 to be moved downward (see arrow162). Of course, since the adjustment pin 88 abuts the rear end 84 of theslot 80, the pulling action (arrow 160) will cause the counterclockwisepivotal movement of the distal end of the rear arm 20 about pivotal axis78.

CA 02282787 1999-09-1718 Finally, turning to Figure 9 of the appended drawings,the operation of the progressive spring assembly 24 will be brieflydescribed. As can be seen from this figure, the proximate portion 46 ofthe rear suspension arm 20 contacts and compresses the contact portion122 of the spring 112 to absorb energy from the movement of thesnowmobile suspension system 10 towards its retracted position toeventually stop this movement before the snowmobile suspension systemreaches its fully retracted position.10 As will be apparent to one skilled in the art, (a) since thedistance 166 between the point of contact of the arm 20 and the spring112 and the center of the spring 112 decreases as the snowmobilesuspension system 10 moves towards its fully retracted position, and (b)since the torsion of the spring 112 increases as the above-noted distancedecreases, the strength of the spring 112 increases progressively as thesnowmobile suspension system 10 nears its fully retracted position. It is to be noted that, in Figure 9, the undersurface 26 ofthe side rail 14 does not present a concave profile since the downwardforce applied by the snowmobile is sufficient to temporarily deform theside rails. Although the present invention has been describedhereinabove by way of preferred embodiments thereof, it can be modified,without departing from the spirit and nature of the subject invention asdefined in the appended claims.

(Source: IFI)

1. A snowmobile suspension system configured tosuspend a drive track to a snowmobile; said suspension systemcomprising:a front suspension arm having a proximate end pivotallymounted to the snowmobile; said front suspension arm having a distalend;a rear suspension arm having a proximate end pivotallymounted to the snowmobile; said rear suspension arm having a distalend;a pair of side rails; each side rail being pivotally mountedto said distal ends of said front and rear suspensions for suspensionmovements between a retracted position where the side rails are adjacentto the snowmobile and an extended position; each said side rail beingmade of composite material and provided with a concave undersurface;anda biasing assembly pivotally mounted to the snowmobileand to both said side rails; said biasing assembly biasing both said siderails towards said extended position.

2. A snowmobile suspension system as recited in claim1, wherein said concave undersurface of each side rail is generally semi-circular.

3. A snowmobile suspension system as recited in claim2, wherein said semi-circular undersurface has a radius of about 400inches.

4. A snowmobile suspension system as recited in claim1, wherein said concave undersurface presents a maximum deflection ofabout 1/4 of an inch.

5. A snowmobile suspension system as recited in claim1, wherein said composite material includes Ultra High Molecular Weight(UHMW) material.

6. A snowmobile suspension system as recited in claim5, wherein said UHMW material contains Ultra High Molecular Weight Polyethylene (UHMW-PE).

7. A snowmobile suspension system as recited in claim1, wherein said front and rear suspension arms are made of compositematerial.

8. A snowmobile suspension system as recited in claim7, wherein said composite material contains Ultra High Molecular Weight Polyethylene (UHMW-PE).

9. A snowmobile suspension system configured tosuspend a drive track to a snowmobile; said suspension systemcomprising:a front suspension arm having a proximate end pivotallymounted to the snowmobile; said front suspension arm having a distalend;a rear suspension arm having a proximate end pivotallymounted to the snowmobile; said rear suspension arm having a distalend;

a pair of side rails; each side rail being pivotally mountedto said distal ends of said front and rear suspensions for suspensionmovements between a retracted position where the side rails are adjacentto the snowmobile and an extended position;a biasing assembly pivotally mounted to the snowmobileand to both said side rails; said biasing assembly biasing both said siderails towards said extended position; anda progressive spring assembly so mounted to said pairof side rails as to contact at least one of said front and rear suspensionarms when said pair of side rails nears said retracted position therebyabsorbing excess energy from said side rails to prevent forceful contactbetween the side rails and the snowmobile.

10. A snowmobile suspension system as recited inclaim 9, wherein said progressive spring assembly is so mounted to saidpair of side rails as to contact said rear suspension arm when said pair ofside rails nears said retracted position.

11. A snowmobile suspension system as recited inclaim 9, wherein said progressive spring assembly includes a firstprogressive spring element mounted to one of said side rails and asecond progressive spring element mounted to the other of said siderails.

12. A snowmobile suspension system as recited inclaim 11, wherein each said progressive spring includes a L-shaped legconfigured and sized to be connected to a respective side rail to preventunwanted movement of said progressive spring.

13. A snowmobile suspension system as recited inclaim 12, wherein each of said progressive spring also includes a straightleg configured and sized to contact said rear suspension arm when saidpair of side rails nears said retracted position.

14. A snowmobile suspension system as recited inclaim 9, wherein said front and rear suspension arms are made of acomposite material.

16. A snowmobile suspension system configured tosuspend a drive track to a snowmobile; said suspension systemcomprising:a front suspension arm having a proximate end pivotallymounted to the snowmobile; said front suspension arm having a distalend;a rear suspension arm having a proximate end pivotallymounted to the snowmobile; said rear suspension arm having a distalend;first and second longitudinal side rails; each longitudinalside rail being pivotally mounted to said distal end of said frontsuspension arm; each longitudinal side rail having a generally transversalrear pivotal axis and a semi-circular channel coaxial with said rear pivotalaxis; said semi-circular channel having opposite ends;a linkage assembly connecting said distal end of saidrear suspension arm to said pair of side rails; said linkage assemblyincluding, for each of said pair of side rails:

an adjustable cam assembly so mounted to said siderail as to pivot about said rear pivotal axis; said camassembly having an off center arm mounting apertureto which said distal end of said rear suspension arm ispivotally mounted; andat least one adjustable abutment element so mountedto said cam assembly as to extend in said semi-circularchannel; said abutment element limiting the pivotalmovements of said cam assembly by abutting said endsof said semi-circular channel; anda biasing assembly pivotally mounted to the snowmobileand to both said side rails; said biasing assembly biasing both said siderails towards an extended position.

17. A snowmobile suspension system as recited in claim16, wherein each said adjustable cam assembly includes first and secondsemi-circular disks so mounted to opposite lateral sides of saidlongitudinal side rails as to pivot about said generally transversal rearpivotal axis.

18. A snowmobile suspension system as recited inclaim 17, wherein each said semi-circular disks includes at least twoapertures aligned with said semi-circular channel, and wherein said atleast one adjustment element includes at least one adjustment pinconfigured and sized to be inserted in said apertures of said disks whileextending in said semi-circular channel.

19. A snowmobile suspension system as recited inclaim 18, wherein said at least two apertures include seven apertures andwherein said at least one adjustment pin includes two adjustment pins.

20. A snowmobile suspension system as recited inclaim 16, wherein said front and rear suspension arms are made of acomposite material.